Although others have shown that the sympathetic nervous system can reduce blood flow through the coronary arteries by stimulation of alpha adrenergic receptors it is not clear that this reduction in coronary blood flow is significant in terms of causing the heart any functional problem. Thus, we investigated this possibility in dogs where we could measure coronary blood flow during intracoronary administration of an alpha adrenergic agonist, methoxamine. We noted that there was coronary vasoconstriction localized to the small coronary arteries and that this coronary vasoconstriction impaired the ability of the heart to contract. These experiments suggest that the sympathetic nervous system can cause vasoconstriction of small coronary arteries which can override the normal autoregulatory system to cause myocardial ischemia and contractile dysfunction.